gmas · February 17, 2014 05:28 · Feb 18, 2013
diff --git a/Ruby.markdown b/Ruby.markdown
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+### Core ideas
+
+* An `Object` is a set of instance variables and a pointer to a 'singleton class'.
+* Properties are looked up in the instance variables, methods are dispatched via the singleton class.
+* `Module` is a subtype of `Object`. A `Module` is a set of methods and an ordered list of zero-or-more 'parent' modules.
+* Module `A` becomes a parent of module `B` via `B.include(A)`.
+* Method lookup works by doing a depth-first right-to-left search of a module tree.
+* `Class` is a subtype of `Module`. A `Class` is a `Module` that can be instantiated.
+* A `Class` has only one 'superclass'. A class includes its superclass as its first parent module for the purposes of method dispatch. A class's singleton class includes the superclass's singleton class as its first parent.
+* The default superclass of all classes is `Object`.
+* `Object` includes the `Kernel` module as a parent.
+
+### Warts
+
+* Although `Class` is a subtype of `Module`, you cannot call `B.include(A)` if `A` is a `Class`.
+* Singleton classes are not really classes. You cannot instantiate or inherit from them.
+
+### Sugar
+
+* `obj.extend(M)` is sugar for `obj.singleton_class.include(M)`.